The Effects of Ion Motion in Very Intense Beam-Driven Plasma Wakefield Accelerators

Recent proposals for using plasma wakefield accelerators in the blowout regime as a component of a linear collider have included very intense driver and accelerating beams, which have densities many times in excess of the ambient plasma density. The electric fields of these beams are widely known to be large enough to completely expel plasma electrons from the beam path; the expelled electrons often attain relativistic velocities in the process. We examine here another aspect of this high-beam density scenario: the motion of ions. In our analysis, for both cylindrically symmetric and flat beams, it is seen that for the proposed "afterburner" scenario the ions completely collapse inside of the electron beam. In this case the ion density is spikes, with a large growth in the beam emittance expected as a result. Particle-in-cell simulations of ion-collapse are presented. Implications of ion motion on the feasibility of the afterburner idea are discussed.